This invention relates to a wellhead connection to casing. The invention also extends to a method of making the wellhead connection.
The lower part of a wellhead is called a casing head or tubing head, hereinafter termed casing head. The casing head is attached to a casing (pipe) and provides a connection to the wellhead equipment located thereabove. The connection must be able to seal well pressure and transmit mechanical loads in any direction. Common connections to a casing are either by threading or by welding. For high temperature service the common connection is welding. The problems with these methods of attaching the casing head are that they may require extensive time and labour, are often expensive, and create the possibility of installation errors. Also, experienced welders may not be available.
Another method is to attach the casing head using a means for gripping the casing pipe with mechanically activated teeth. A seal between the casing head and the casing pipe is then provided separately, usually in the form of an elastomeric seal ring (for example an O-ring) located in the casing head above a mechanical gripping mechanism. Such an assembly is well known in the industry and is described in, for example, U.S. Pat. No. 4,799,714 issued to Collet, U.S. Pat. No. 5,332,043 issued to Ferguson, and Canadian Patent 2,015,966 issued to Anderson et al. Each of these patents describes a known method for mechanically attaching the casing head to the surface casing. These patents disclose the use of conical slip segments which surround the casing pipe, each slip segment being provided with a plurality of grooves on their straight inside surface (casing pipe-contacting surface) that act as teeth that bite into the outer surface of the casing. A slip housing, or actuation sleeve, with a reverse conical mating surface to the conical surface on the outside of the slip segments, is then driven against the slip segments (or the slip segments are driven against the sleeve/housing). This forces the slip segments against the surface casing pipe causing the grooves to frictionally grip (or the teeth to bite into) the casing pipe, and thus to secure the casing pipe to the casing head. These slip segments are commonly referred to as “slips” and the system is commonly described as a slip lock casing connector, or slip connector.
The slip lock casing connector has advantages over the previously described casing connectors. These include reduced installation time compared to welding, and unlike a threaded connection, proper orientation of the head can be achieved.
While one or more elastomeric seals, such as O-ring seals are typically used to seal the central bore of the casing head to the surface casing, in high temperature and/or corrosive environments, these seals can fail. In those applications, a metal seal to the casing pipe is preferred. Several examples of metal seals in casing heads are shown in the prior art. For example, U.S. Pat. No. 5,158,326 to Anderson et al., describes a casing head system which includes a pair of metal seal rings above slip lock connectors. Both the slips and the metal seal are actuated with a dual acting hydraulic piston, which when activated in different directions, has the effect of energizing the seal rings and the slip connectors. This is a complex device, in which the slip connectors are energized opposite to the normal direction, with the casing moving upwardly against the downwardly moving slips (normally the casing pulls down against the upwardly moving slips in casing slip lock connectors). During this movement to energize the slips, there is opportunity for the casing to move downwardly, thus damaging the metal seal. Ideally, a metal seal, to remain intact, should move only radially inwardly to seal the casing, with no vertical movement.
U.S. Pat. No. 5,135,266 to Bridges et al., shows a combination slip lock connector and seal assembly for a casing head. An annular metal slip member has upper and lower conical sections which co-operate with conical sections of upper and lower wellhead housing members such that the slip member is wedged and thus moves radially inwardly as the upper and lower wellhead members are bolted together. Elastomeric seals are provided at the upper and lower ends of the annular slip member. As noted in the patent, care must be taken not to apply too much radial force on installation, since this can cause the slip member to crush the casing. As with the Anderson et al. patent, the seals and the slips in the Bridges et al. patent are simultaneously energized, making it difficult to prevent vertical movement at the seals.
A wellhead connection is needed which takes advantage of slip lock connectors, while also providing for a reliable metal seal to the casing such that the seal can be energized without vertical movement being imparted from the slip energizing step.